1. Field of the Invention
This invention relates generally to protective relay apparatus for protecting ac electrical power transmission lines, and more specifically, to ultra-high-speed relays.
2. Description of the Prior Art
Three-phase ac electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits or drops in shunt resistance that could cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault is caused by lightning-induced flashover from a transmission line to ground or between transmission line conductors. Under such a faulted condition, line currents can increase to several times the normal value thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment.
To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in 0.1 to 0.5 seconds. The isolation time limit must allow for the operation of large circuit breakers interrupting up to 80,000 A. and completion of back-up operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, the location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which continuously monitor ac voltages and currents, to locate line faults and initiate isolation via tripping of the appropriate circuit breakers.
The direction and distance to a fault with reference to a measuring location on a transmission line is usually determined with the aid of distance relays. These relays incorporate electromechanical or electronic elements requiring substantially sinusoidal power-frequency input signals to function correctly. When a fault occurs on the transmission line, there is a deviation in the power-frequency current and voltage signals and transient traveling waves are also generated. Since the power-frequency signals as distorted by the traveling wave are not suitable for detection by a distance relay, operation of the protective relay must await decay of the distortion effects, i.e., the traveling waves. This decay is a comparatively slow process. Alternatively, frequency filters may be used to filter the effects of the transient traveling waves thereby enabling the power frequency components to be evaluated by the protective relay at an earlier time. By evaluating the change in the power-frequency signals after the fault as compared to the steady-state values before the fault, distance relays can make independent or zone 1 trip decisions.
One prior art approach for making such zone 1 trip decisions, disclosed in U.S. Pat. No. 4,287,547, plots the trajectory of the fault-generated voltage and current deviations as a function of time on a deviation plane, with the voltage deviation on the X-axis and the current deviation on the Y-axis. The plot defines an elliptical orbit with clockwise rotation indicating a fault in the forward direction. Zone 1 threshold boundaries are established in each quadrant and if the trajectory crosses a forward boundary, it is known that the fault is in the forward direction and within the protected zone. This zone 1 trip decision, however, is complicated by the fact that the position and shape of the elliptical orbit is a function of both the fault inception angle .gamma. and the source impedance X.sub.s. Thus, it is extremely difficult to set the boundary thresholds for zone 1 trips.
U.S. Pat. No. 4,371,907, which is assigned to the same assignee as the present invention solves the practical problems associated with the elliptical trajectory concept by differentiating the current deviation signal and using this differentiated signal as the Y-axis coordinate. Alternatively, the voltage deviation signal may be integrated. For making independent or zone 1 trip decisions, the traveling wave response is filtered from the deviation signals. Instead of an ellipse having a rotational direction responsive to fault direction, a forward fault produces a straight line trajectory on this transformed deviation plane with the straight line crossing the origin between the second and fourth quadrants for a forward direction fault.
Unlike the elliptical orbit, where position and shape is dependent upon the fault inception angle .gamma. and the source impedance X.sub.s, the straight-line trajectories are unaffected by fault inception angle. Only the slope of the straight line trajectory changes with source impedance. Thus, the boundary thresholds for detecting a zone 1 fault need not be set for the "worst case," i.e., to accommodate the full range of possible X.sub.s and .gamma. values, as would be required for the prior art elliptical trajectory approach. The straight-line trajectory prior art protective relay may therefore be set for a much greater percentage of the protected line, and it detects and clears more zone 1 faults better than a system using an elliptical orbit concept.
The present invention represents an important improvement over the above-referenced U.S. Pat. No. (4,371,907). The boundary thresholds of the prior art relay are represented by fixed straight lines established on the basis of the expected peak amplitudes of the differentiated current deviation signal and the voltage-deviation signal. The present invention discloses a variable threshold boundary to provide faster fault detection. That is, the threshold boundary varies over time in accord with the straight line trajectory, and therefore tripping does not have to await crossing of a fixed straight line threshold by a peak value of the differentiated current-deviation signal or of the voltage-deviation signal.